Why does the GNU Project need so many Lisps?
It doesn't need so many Lisps, but it has so many Lisps because there is something wrong with Lispers. Lispers are more interested in making new Lisp implementations than in maintaining existing implementations. That's why there is a proliferation of half-baked, incomplete, slow, and/or generally inferior Lisp implementations. You can also see this in the library ecosystem for Lisp (especially Scheme), where the same functions get written over and over again because it is "so easy to do so".
The GNU Project does not have 5 C compilers, 5 C++ compilers, 5 Fortran compilers, and 5 Smalltalk implementations. It doesn't need 5 Lisp implementations either.